Spider

Morphology of a spider: arthropod
animal with eight legs and an unsegmented body.First leg: first locomotive appendage of a spider in relation
to its head.Nail: pointed nail.Second leg: second locomotive appendage of a spider in relation
to its head.Abdomen: rear part of a spider's body.Third leg: third locomotive appendage of a spider in relation
to its head.Fourth leg: fourth locomotive appendage of a spider in relation
to its head.Spinneret: opening through which the spider emits its silk.Tarsus: last part of the leg of a spider.Metatarsus: fourth part of the leg.Tibia: third part of the leg.Patella: second part of the leg.Femur: first part of the leg.Eye: sight organ of a spider.Chelicera: pair of venomous hooks on the spider's head.Pedipalp: tactile foot of a spider.

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EN : Spiders

FR : Araignées

ES : Arañas

Spiders are predatory invertebrate animals
that have two body segments, eight legs, no chewing mouth parts and
no wings. They are classified in the order Araneae, one of several orders
within the larger class of arachnids, a group that also contains scorpions,
whip scorpions, mites, ticks, and opiliones. The study of spiders is
called arachnology. All spiders produce silk, a thin, strong protein
strand extruded by the spider from spinnerets most commonly found on
the end of the abdomen. Many species use it to trap insects in webs,
although there are also many species that hunt freely. Silk can be used
to aid in climbing, form smooth walls for burrows, build egg sacs, wrap
prey, and temporarily hold sperm, among other applications.

All spiders except those in the families
Uloboridae and Holarchaeidae, and in the suborder Mesothelae (together
about 350 species) can inject venom to protect themselves or to kill
prey. Only about 200 species, however, have bites that can pose health
problems to humans. Many larger species' bites may be quite painful,
but will not produce lasting health concerns. Spiders are found all
over the world, from the tropics to the Arctic, living underwater in
silken domes they supply with air, and on the tops of mountains. In
1973, Skylab 3 took two spiders into space to test their web-spinning
capabilities in zero gravity.

Spiders are chelicerates and therefore
arthropods. As arthropods they have: segmented bodies with jointed limbs,
all covered in a cuticle made of chitin and proteins; heads that are
composed of several segments that fuse during the development of the
embryo. Being chelicerates, their bodies consist of two tagmata, sets
of segments that serve similar functions: the foremost one, called the
cephalothorax or prosoma, is a complete fusion of the segments that
in an insect would form two separate tagmata, the head and thorax; the
rear tagma is called the abdomen or opisthosoma. The pattern of segment
fusion that forms chelicerates' heads is unique among arthropods, and
what would normally be the first head segment disappears at an early
stage of development, so that chelicerates lack the antennae typical
of most arthropods. In fact chelicerates' only appendages ahead of the
mouth are a pair of chelicerae, and they lack anything that would function
directly as "jaws". The first appendages behind the mouth
are called pedipalps, and serve different functions within different
groups of chelicerates.

Spiders and scorpions are members of one
chelicerate group, the arachnids. While scorpions' chelicerae are generally
a modest pair of claws that they use in feeding, spiders' terminate
in fangs that are are generally venomous, and fold away behind the upper
sections while not in use, while the upper sections generally have thick
"beards" that filter solid lumps out of their food, as spiders
can take only liquid food; On the other hand scorpions' pedipalps generally
form large claws for capturing prey, while those of spiders are fairly
small sensors whose bases also act as an extension of the mouth; in
addition those of male spiders have enlarged last sections used for
sperm transfer.

In spiders the cephalothorax and abdomen
are joined by a small, cylindrical pedicel, which enables the abdomen
to move independently when producing silk. The upper surface of the
cephalothorax is covered by a single, convex carapace while the underside
is covered by two rather flat plates. The abdomen is soft and egg-shaped.
It shows no sign of segmentation, except that the primitive Mesothelae,
whose living members are the Liphistiidae, have segmented plates on
the upper surface.

Spiders, unlike insects, have only two
body segments (tagmata) instead of three: a fused head and thorax (called
a cephalothorax or prosoma) and an abdomen (called the opisthosoma).
The exception to this rule are the assassin spiders, whose cephalothorax
seems to be almost divided into two independent units. The abdomen and
cephalothorax are connected with a thin waist called the pedicle or
the pregenital somite, a trait that allows the spider to move the abdomen
in all directions. The pedicle (waist) is actually the last segment
(somite) of the cephalothorax and is lost in most other members of the
Arachnida (in scorpions it is only detectable in the embryos).

Spiders occur in a large range of sizes.
The smallest, dwarf spiders of the subfamily Erigoninae, are less than
1 mm (about .05 inches) in body length. The largest and heaviest spiders
occur among tarantulas, which can have body lengths up to 90 mm (about
3.5 inches) and leg spans up to 250 mm (about 10 inches). Only three
classes of pigment (ommochromes, bilins and guanine) have been identified
in spiders, although other pigments have been detected but not yet characterized.
Melanins, carotenoids and pterins, very common in other animals, are
apparently absent. In some species the exocuticle of the legs and prosoma
is modified by a tanning process, resulting in brown coloration. Bilins
are found for example in Micrommata virescens, resulting in its green
color. Guanine is responsible for the white markings of the European
garden spider Araneus diadematus. It is in many species accumulated
in specialized cells called guanocytes. In genera such as Tetragnatha,
Leucauge, Argyrodes or Theridiosoma, guanine creates their silvery appearance.
While guanine is originally an end-product of protein metabolism, its
excretion can be blocked in spiders, leading to an increase in its storage.
Structural colors occur in some species, which are the result of the
diffraction, scattering or interference of light, for example by modified
setae or scales. The white prosoma of Argiope results from hairs reflecting
the light, Lycosa and Josa both have areas of modified cuticle that
act as light reflectors.

The best-known method of prey capture
is by means of sticky webs. Varying placement of webs allows different
species of spider to trap different insects in the same area, for example
flat horizontal webs trap insects that fly up from vegetation underneath
while flat vertical webs trap insects in horizontal flight. Web-building
spiders have poor vision, but are extremely sensitive to vibrations.
A few spiders use the surfaces of lakes and ponds as "webs",
detecting trapped insects by the vibrations that these cause while struggling.

Net-casting spiders weave only small webs
but then manipulate them to trap prey. Those of the genus Hyptiotes
and the family Theridiosomatidae stretch their webs and then release
them when prey strike them, but do not actively move their webs. Those
of the family Deinopidae weave even smaller webs, hold them outstretched
between their first two pairs of legs, and lunge and push the webs as
much as twice their own body length to trap prey, and this move may
increase the webs' area by a factor of up to ten. Experiments have shown
that Deinopis spinosus has two different techniques for trapping prey:
strikes over its back to catch flying insects, whose vibrations it detects;
and forward strikes to catch ground-walking prey that it sees. These
two techniques have also been observed in other deinopids. Walking insects
form most of the prey of most deinopids, but one population of Deinopis
subrufus appears to live mainly on tipulid flies that they catch with
the backwards strike.

Mature female bolas spiders of the genus
Mastophora build "webs" that consist of only a single "trapeze
line", which they patrol. They also construct a bolas made of a single
thread, tipped with a large ball of very wet sticky silk. They emit chemicals
that resemble the pheromones of moths, and then swing the bolas at the
moths. Although they miss on about 50% of strikes, they catch about the
same weight of insects per night as web-weaving spiders of similar size.
The spiders eat the bolas if they have not made a kill in about 30 minutes,
rest for a while, and then make new bolas. Wolf spiders, jumping spiders,
fishing spiders and some crab spiders capture prey by chasing it, and
rely mainly on vision to locate prey. Others, including many crab spiders,
are ambush predators. A few species that prey on bees, which see ultraviolet,
can adjust their ultraviolet reflectance to match the flowers in which
they are lurking.

Portia uses both webs and cunning, versatile
tactics to overcome prey. Some jumping spiders of the genus Portia hunt
in ways that seem intelligent. When stalking web-building spiders, some
of which would be dangerous opponents, they lure them out by vibrating
the webs to mimic the struggle of a trapped insect or the courtship
signals of a male spider, or approach along an overhanging twig or rock
and abseil down a silk thread to kill the prey. Laboratory studies show
that Portia learns very quickly how to overcome web-building spiders
that neither it nor its evolutionary ancestors would have met in the
wild. While some of these behaviors are clearly instinctive, experiments
have shown that Portias instinctive tactics are only starting
points for a trial-and-error approach from which these spiders learn
very quickly. However they seem to be relatively slow "thinkers",
which is not surprising as their brains are vastly smaller than those
of mammalian predators. Once within biting range, Portias use different
combat tactics against different prey spiders. On the other hand they
simply stalk and rush unarmed prey such as flies, and also capture prey
by means of sticky webs.

Ant-mimicking spiders face several challenges:
they generally develop slimmer abdomens and false "waists"
in the cephalothorax to mimic the three distinct regions (tagmata) of
an ant's body; they wave the first pair of legs in form to their heads
to mimic antennae, which spiders lack, and to conceal the fact that
they have but eight legs rather than six; they develop large color patches
round one pair of eyes to disguise the fact that they generally have
eight simple eyes, while ants have two compound eyes; they cover their
bodies with reflective hairs to resemble the shiny bodies of ants. In
some spider species males and females mimic different ant species, as
female spiders are usually much larger than males. Ant-mimicking spiders
also modify their behavior to resemble that of the target species of
ant, for example many adopt a zig-zag pattern of movement, ant-mimicking
jumping spiders avoid jumping, and spiders of the genus Synemosyna walk
on the outer edges of leaves in the same way as Pseudomyrmex. Ant-mimicry
in many spiders and other arthropods may be for protection from predators
that hunt by sight, including birds, lizards and spiders. However several
ant-mimicking spiders prey either on ants or on the ants "livestock"
such as aphids. When at rest the ant-mimicking crab spider Amyciaea
does not closely resemble Oecophylla, but while hunting it imitates
the behavior of a dying ant to attract worker ants. After a kill some
spiders hold their victims between themselves and large groups of ants
to avoid being attacked.

While spiders are generalist predators,
in actuality their different methods of prey capture often determine
the type of prey taken. Thus web-building spiders rarely capture caterpillars,
and crab spiders that ambush prey in flowers capture more bees, butterflies
and some flies than other insects. Groups of families that tend to take
certain types of prey because of their prey capture methods are often
called guilds. A few spiders are more specialized in their prey capture.
Dysdera captures and eats sowbugs, pillbugs and beetles, while pirate
spiders eat only other spiders. Bolas spiders in the family Araneidae
use sex pheromone analogs to capture only the males of certain moth
species. Despite their generally broad prey ranges, spiders are one
of the most important links in the regulation of the populations of
insects.